Single Stage and Scalable Serializer

ABSTRACT

According to an exemplary embodiment, a serializer includes upper and lower shift registers configured to perform a load function where parallel input data is loaded from a parallel input bus and a shift function where the parallel input data is shifted to an output register. The upper shift register is configured to perform the load function while the lower shift register performs the shift function, and the lower shift register is configured to perform the load function while the upper shift register performs the shift function. An output register is configured to alternately receive the parallel input data from the upper shift register and the parallel input data from the lower shift register. The upper and lower shift registers and the output register can comprise scan flip-flops.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally in the field of electrical circuitsand systems. More particularly, the invention relates to electricalcircuits and systems utilized in conversion of parallel data into serialdata.

2. Background Art

A serializer can be used to convert low-speed data on a parallel inputbus into high-speed data on a serial output. The parallel input busincludes multiple channels, which are sequentially selected by theserializer so as to transfer the data from the parallel input bus to theserial output. Conventional serializers typically include variousfeatures, which result in a slow conversion speed, high powerconsumption, and/or large size. Furthermore, many conventionalserializer designs are not scalable and require substantial redesignwhen the number of channels of the parallel input bus is varied.

Conventional serializers typically convert the low-speed data into thehigh-speed data in multiple stages. For example, the parallel input buscan have sixteen channels where the low-speed data is converted by onestage into mid-speed data on four channels, which is then converted intothe high-speed data on the serial output. However, designing aserializer with multiple stages can be result in a larger, slower,and/or more complicated design. Furthermore, these designs are typicallynot scalable, consume high power, and/or can be undesirably large.Conventional serializers may also place synchronization or re-timeflip-flops and multiplexers or other combinational logic in a dataconversion path of the serializer, thereby reducing conversion speed andincreasing the size of the serializer. Some serializers may require aphase shifter to generate a signal for controlling selection of thechannels in the serializer. However, the phase shifter can be difficultto design and is typically not scalable. Also, some serializers mayutilize current mode logic (CML) for the conversion. While CML may befast, additional circuitry may be required to convert from acomplementary metal oxide semiconductor (CMOS) signal to a CML signal,thereby increasing the size of the serializer. Furthermore, CML requiresa constant bias current resulting in high power consumption.

Thus, there is a need in the art for a serializer that can have highconversion speed, low power consumption, and small size while beingscalable to accommodate variation in the number of channels of theparallel input.

SUMMARY OF THE INVENTION

A single stage and scalable serializer, substantially as shown in and/ordescribed in connection with at least one of the figures, and as setforth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary serializer, according to one embodimentof the invention.

FIG. 2 illustrates an exemplary control circuit of a serializer,according to one embodiment of the invention.

FIG. 3 illustrates exemplary upper and lower shift registers and anoutput register of a serializer, according to one embodiment of theinvention.

FIG. 4 illustrates exemplary waveforms of a serializer, according to oneembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a single stage and scalableserializer. The following description contains specific informationpertaining to the implementation of the present invention. One skilledin the art will recognize that the present invention may be implementedin a manner different from that specifically discussed in the presentapplication. Moreover, some of the specific details of the invention arenot discussed in order to not obscure the invention. The specificdetails not described in the present application are within theknowledge of a person of ordinary skill in the art.

The drawings in the present application and their accompanying detaileddescription are directed to merely exemplary embodiments of theinvention. To maintain brevity, other embodiments of the invention thatuse the principles of the present invention arc not specificallydescribed in the present application and are not specificallyillustrated by the present drawings.

FIG. 1 illustrates serializer 100, according to one embodiment of theinvention. Serializer 100 includes control circuit 102, upper shiftregister 104, lower shift register 106, output register 108, parallelinput bus 110, and serial output 112.

In FIG. 1, parallel input bus 110 comprises a plurality of channels andserializer 100 is configured to convert data from the plurality ofchannels into serial output 112. For example, in FIG. 1, serializer 100is configured to convert parallel input data P_(i1) and P_(i2) intoserial output data S_(o). As an example, parallel input bus 110 can haveeight channels and serializer 100 can convert the eight channels intoserial output 112 comprising a single channel. Parallel input dataP_(i1) and P_(i2) each comprise one bit per channel of parallel inputbus 110. Thus, in the above example, parallel input data P_(i1) andP_(i2) each comprise eight bits.

In serializer 100, upper shift register 104 is configured to performload function 114 where parallel input data P_(i1) is loaded fromparallel input bus 110 and shift function 116 where parallel input dataP_(i1) is shifted to output register 108. As described above, parallelinput data P_(i1) comprises one bit per channel of parallel input bus110 and those bits are loaded in parallel into upper shift register 104by load function 114. Output register 108 can receive parallel inputdata P_(i1) from upper shift register 104 as shifted (serialized) dataSH₁, which comprises parallel input data P_(i1) after being shiftedthrough upper shift register 104 by shift function 116. Output register108 can provide shifted (serialized) data SH₁ in serial output dataS_(o) at serial output 112.

Similarly, lower shift register 106 is configured to perform loadfunction 118 where parallel input data P_(i2) is loaded from parallelinput bus 110 and shift function 120 where parallel input data P_(i2) isshifted to output register 108. As described above, parallel input dataP_(i2) comprises one bit per channel of parallel input bus 110 and thosebits arc loaded in parallel into lower shift register 106 by loadfunction 118. Output register 108 can receive parallel input data P_(i2)from lower shift register 106 as shifted (serialized) data SH₂, whichcomprises parallel input data P_(i2) after being shifted through lowershift register 106 by shift function 120. Output register 108 canprovide shifted (serialized) data SH₂ in serial output data S_(o) atserial output 112.

In serializer 100, output register 108 is configured to alternatelyreceive parallel input data P_(i1) from upper shift register 104 asshifted (serialized) data SH₁ and parallel input data P_(i2) from lowershift register 106 as shifted (serialized) data SH₂ to provide serialoutput data S_(o) at serial output 112. In the present embodiment,output register 108 is configured to alternate between receivingparallel input data P_(i1) from upper shift register 104 as shifted(serialized) data SH₁ and parallel input data P_(i2) from lower shiftregister 106 as shifted (serialized) data SH₁. Thus, serial output dataS_(o) at serial output 112 comprises a stream of serialized dataalternating between parallel input data P_(i1) and parallel input dataP_(i2).

As shown in FIG. 1, serializer 100 receives load clock CLK_(L) and shiftclock CLK_(SH) as inputs to convert parallel input data P_(i1) andP_(i2) into serial output data S_(o). Control circuit 102 receives loadclock CLK_(L) and shift clock CLK_(SH) to manage upper shift register104 and lower shift register 106 to convert parallel input data P_(i1)and parallel input data P_(i2) into serial output data S_(o). Forexample, control circuit 102 utilizes load clock CLK_(L) to setfrequency f_(L) of load functions 114 and 118. More particularly, loadfunctions 114 and 118 are performed by upper and lower shift registers104 and 106 respectively at frequency f_(L), which in the presentembodiment is the same frequency as load clock CLK_(L). In the presentembodiment, load clock CLK_(L) is a 1 GHz clock. Also, control circuit102 utilizes shift clock CLK_(SH) to set frequency f_(SH) of shiftfunctions 116 and 120. More particularly, shift functions 116 and 120are performed by upper and lower shift registers 104 and 106respectively at frequency f_(SH), which in the present embodiment is thesame frequency as shift clock CLK_(SH). In the present embodiment, shiftclock CLK_(SH) is an 8 GHz clock.

Control circuit 102 provides function select clock CLK_(FS1) andfunction control clock CLK_(FC1) to upper shift register 104 to manageupper shift register 104 and provides function select clock CLK_(FS2)and function control clock CLK_(FC2) to lower shift register 106 tomanage lower shift register 106 so as to convert parallel input dataP_(i1) and parallel input data P_(i2) into serial output data S_(o).

Upper shift register 104 is configured to select between performing loadfunction 114 and shift function 116 responsive to function select clockCLK_(FS1). For example, in the present embodiment, when function selectclock CLK_(FS1) is low, upper shift register 104 is configured toperform load function 114 and when function select clock CLK_(FS1) ishigh, upper shift register 104 is configured to perform shift function116. Thus, in the present embodiment, upper shift register 104 isconfigured to repeatedly perform load function 114 and shift function116 at a frequency of function select clock CLK_(FS1). In the presentembodiment, function select clock CLK_(FS1) has half the frequency ofload clock CLK_(L) and is a 500 MHz clock signal.

Similarly, lower shift register 106 is configured to select betweenperforming load function 118 and shift function 120 responsive tofunction select clock CLK_(FS2). For example, in the present embodiment,when function select clock CLK_(FS2) is low, lower shift register 106 isconfigured to perform load function 118 and when function select clockCLK_(FS2) is high, lower shift register 106 is configured to performshift function 120. Thus, in the present embodiment, lower shiftregister 106 is configured to repeatedly perform load function 118 andshift function 120 at a frequency of function select clock CLK_(FS2). Inthe present embodiment, function select clock CLK_(FS2) has half thefrequency of load clock CLK_(L) and is a 500 MHz clock signal.

Function select clock CLK_(FS2) is the inverse of function select clockCLK_(FS1). Thus, while upper shift register 104 is performing loadfunction 114 on parallel input data P_(i1), lower shift register 106 isperforming shift function 120 on parallel input data P_(i2). Similarly,while upper shift register 104 is performing shift function 116 onparallel input data P_(i1), lower shift register 106 is performing loadfunction 118 on parallel input data P_(i2). As such, upper shiftregister 104 is configured to perform load function 114 while lowershift register 106 performs shift function 120 and lower shift register106 is configured to perform load function 118 while upper shiftregister 104 performs shift function 116.

As shown in FIG. 1, in the present embodiment, upper shift register 104is configured to perform load and shift functions 114 and 116 atrespective frequencies based on function control clock CLK_(FC1.) Moreparticularly, upper shift register 104 is configured to perform loadfunction 114 at frequency f_(L) of function control clock CLK_(FC1) andto perform shift function 116 at frequency f_(SH) of function controlclock CLK_(FC1). In the present embodiment, function control clockCLK_(FC1) alternates between having frequency f_(L) and frequency f_(SH)at the frequency of function select clock CLK_(FS1) and thus, willrepeat this pattern at half the frequency of frequency f_(L), which isthe same as the frequency of load clock CLK_(L) (e.g. every 500 MHzfunction control clock CLK_(FC1) will repeat having frequency f_(L)followed by having frequency f_(SH)).

Similarly, lower shift register 106 is configured to perform load andshift functions 118 and 120 at respective frequencies based on functioncontrol clock CLK_(FC2.) More particularly, lower shift register 106 isconfigured to perform load function 118 at frequency f_(L) of functioncontrol clock CLK_(FC2) and to perform shift function 120 at frequencyf_(SH) of function control clock CLK_(FC2). In the present embodiment,function control clock CLK_(FC2) alternates between having frequencyf_(L) and frequency f_(SH) at the frequency of function select clockCLK_(FS2) and thus, will repeat this pattern at half the frequency offrequency f_(L), which is the same as the frequency of load clockCLK_(L) (e.g. every 500 MHz function control clock CLK_(FC2) will repeathaving frequency f_(SH) followed by having frequency f_(L)).

Also in FIG. 1, control circuit 102 is configured to provide outputselect clock CLK_(OS) to output register 108 so as to manage outputregister 108. Output register 108 is configured to alternately receiveparallel input data P_(i1) from upper shift register 104 as shifted(serialized) data SH₁ and parallel input data P_(i2) from lower shiftregister 106 as shifted (serialized) data SH₂ on output select clockCLK_(OS). In the present embodiment, when output select clock CLK_(OS)is low, output register 108 is configured to receive parallel input dataP_(i1) from upper shift register 104 and when output select clockCLK_(OS) is high, output register 108 is configured to receive parallelinput data P_(i2) from lower shift register 106. Thus, in the presentembodiment, output register 108 can be configured to repeatedlyalternate between receiving parallel input data P_(i1) from upper shiftregister 104 as shifted (serialized) data SH₂ and parallel input dataP_(i2) from lower shift register 106 as shifted (serialized) data SH₂ ata frequency of output select clock CLK_(OS).

Output register 108 is configured to receive upper shift register 104when upper shift register 104 is performing shift function 116 and isconfigured to receive lower shift register 106 when lower shift register106 is performing shift function 120. Thus, output select clock CLK_(OS)can be equal to one of function select clocks CLK_(FS1) and CLK_(FS2.)For example, in the present embodiment, output select clock CLK_(OS) isequal to function select clock CLK_(FS2). Thus, output register 108 isconfigured to select between receiving parallel input data P_(i1) fromupper shift register 104 and parallel input data P_(i2) from lower shiftregister 106 responsive to one of function select clocks CLK_(FS1) andCLK_(FS2).

Also in the present embodiment, output register 108 receives shift clockCLK_(SH). Output register 108 is configured to receive parallel inputdata P_(i1) from upper shift register 104 as shifted (serialized) dataSH₁ and parallel input data P_(i2) from lower shift register 106 asshifted (serialized) data SH₂ and output serial output data S_(o) at afrequency based on shift clock CLK_(SH). More particularly, outputregister 108 is configured to receive parallel input data P_(i1) fromupper shift register 104 at a frequency of shift clock CLK_(SH) andreceive parallel input data P_(i2) from lower shift register 106 at afrequency of shift clock CLK_(SH) to output serial output data S_(o) ata frequency of shift clock CLK_(SH). In the present example, shift clockCLK_(SH) has frequency f_(SH). As described above, upper and lowerregisters 104 and 106 are configured to perform shift functions atfrequency f_(SH) of shift clock CLK_(SH). Thus, output register 108 isconfigured to alternately receive parallel input data p_(i1) as shifted(serialized) data SH₁ from upper shift register 104 and parallel inputdata P_(i2) from lower shift register 106 as shifted (serialized) dataSH₂ at frequency f_(SH) of shift clock CLK_(SH). The received parallelinput data p_(i1) and parallel input data p_(i2) can be provided inserial output data S_(o) at frequency f_(SH) of shift clock CLK_(SH).Thus, in the present embodiment, shift clock CLK_(SH) can correspond tothe conversion speed of serializer 100, which as one example can be 8GHz. Serial output data S_(o) can be provided to a digital to analogconverter (DAC) at 8 GHz, which is not shown in FIG. 1.

Conventional serializers typically convert low-speed data on a parallelinput bus into high-speed data on a serial output in multiple stages.For example, conventionally parallel input bus 110 may have sixteenchannels converted by one stage into four channels, which are thenconverted into serial output 112. Advantageously, in accordance withvarious embodiments of the present invention, serializer 100 can convertparallel input data P_(i1) and P_(i2) into serial output data S_(o) in asingle stage. As such, based on the above example, parallel input dataP_(i1) and P_(i2) can each be 16 bits (one bit per channel of parallelinput bus 110), which are converted into serial output data S_(o), whichcomprises a one bit data stream, in a single stage. It will beappreciated that parallel input bus 110 is not limited to 16 channels.As other specific examples, parallel input bus 110 can have 32 channelsand parallel input data P_(i1) and P_(i2) can each be 32 bits orparallel input bus 110 can have 64 channels and parallel input dataP_(i1) and P_(i2) can each be 64 bits.

Thus, as described above, serializer 100 does not require componentsneeded to implement multiple stages. For example, serializer 100 doesnot require multiplexers or other combinational logic in the data pathbetween parallel input bus 110 and serial output 112, which would reduceconversion speed of serializer 100 and does not require complex controlcircuitry to manage the multiple stages. As such, serializer 100 can besmall and have low power consumption. Furthermore, serializer 100 canprevent redesign of the combinational logic and complex controlcircuitry, which may be required when the number of channels of parallelinput bus 110 is varied. Thus, serializer 100 can be easily scalablethereby reducing design cost.

FIG. 2 illustrates control circuit 202, according to one embodiment ofthe invention. Control circuit 202 corresponds to control circuit 102 ofserializer 100 in FIG. 1. Thus, shift clock CLK_(SH), load clockCLK_(L), function select clocks CLK_(FS1) and CLK_(FS2,) functioncontrol clocks CLK_(FC1) and CLK_(FC2), and output select clock CLK_(OS)in FIG. 2 correspond respectively to shift clock CLK_(SH), load clockCLK_(L), function select clocks CLK_(FS1) and CLK_(FS2,) functioncontrol clocks CLK_(FC1) and CLK_(FC2), and output select clock CLK_(OS)in FIG. 1.

Control circuit 202 includes function controllers 222 and 224, inverter226, and frequency divider 228. Control circuit 202 is configured togenerate function select clocks CLK_(FS1) and CLK_(FS2,) functioncontrol clocks CLK_(FC1) and CLK_(FC2), and output select clock CLK_(OS)from shift clock CLK_(SH) and load clock CLK_(L). As described above, inthe present embodiment, in FIG. 1, shift functions 116 and 120 areperformed by upper and lower shift registers 104 and 106 respectively atfrequency f_(SH), which is the frequency of shift clock CLK_(SH). Alsodescribed above, in the present embodiment, in FIG. 1, load functions114 and 118 are performed by upper and lower shift registers 104 and 106respectively at frequency f_(L), which is the frequency of load clockCLK_(L).

In the present embodiment, frequency f_(SH) is equal to a frequencyrequired to shift one bit of parallel input data P_(i1) and P_(i2). Alsoin the present embodiment, frequency f_(L) is less than or equal tofrequency f_(SH) divided by the number of channels in parallel input bus110. Thus, shift function 116, for example, can shift each and every bitof parallel input data P_(i1) to output register 108 within one cycle ofload clock CLK_(L). As a specific example, in the present embodiment,frequency f_(SH) is 8 GHz and frequency f_(L) is 1 GHz. Parallel inputbus 110 comprises eight channels and frequency f_(L) has one-eighth thefrequency of frequency f_(SH). As such, shift function 116 can beperformed to shift the eight bits of parallel input data P_(i1) withinone cycle of load clock CLK_(L). In some embodiments, load clock CLK_(L)can easily be generated from shift clock CLK_(SH) using, for example, afrequency divider.

In control circuit 202, function controller 222 is configured togenerate function control clock CLK_(FC1) by selecting between loadclock CLK_(L) and shift clock CLK_(SH). Similarly, function controller224 is configured to generate function control clock CLK_(FC2) byselecting between load clock CLK_(L) and shift clock CLK_(SH). Asdescribed above, upper shift register 104 is configured to perform loadfunction 114 at frequency f_(L) of function control clock CLK_(FC1) andto perform shift function 116 at frequency f_(SH) of function controlclock CLK_(FC1). Similarly, lower shift register 106 is configured toperform load function 118 at frequency f_(L) of function control clockCLK_(FC1) and to perform shift function 120 at frequency f_(SH) offunction control clock CLK_(FC1). Thus, function controllers 222 and 224are configured to select load clock CLK_(L) to control load functions114 and 118 and to select shift clock CLK_(SH) to control shiftfunctions 116 and 120. As such, upper shift register 104 is configuredto perform load function 114 and shift function 116 by selectivelyreceiving load clock CLK_(L) and shift clock CLK_(SH) and lower shiftregister 106 is configured to perform load function 118 and shiftfunction 120 by selectively receiving load clock CLK_(L) and shift clockCLK_(SH).

In the present embodiment, function controllers 222 and 224 each are amultiplexer configured to select between load clock CLK_(L) and shiftclock CLK_(SH). As shown in FIG. 2, function controllers 222 and 224 areconfigured to select between load clock CLK_(L) and shift clock CLK_(SH)based on function select clocks CLK_(FS1) and CLK_(FS2) respectively. Asdescribed above, function select clock CLK_(FS2) is the inverse offunction select clock CLK_(FS1). As shown in FIG. 2, in the presentembodiment, inverter 226 generates function select clock CLK_(FS1) fromfunction select clock CLK_(FS2). Thus, when function controller 222 isselecting shift clock CLK_(SH), function controller 224 is selectingload clock CLK_(L) and when function controller 222 is selecting loadclock CLK_(L), function controller 224 is selecting shift clockCLK_(SH).

Also in the present embodiment, upper and lower shift registers 104 and106 in FIG. 1 is each configured to perform one respective load function114 and 118 and one respective shift function 116 and 120 per cycle ofrespective function select clocks CLK_(FS1) and CLK_(FS2). In theexample used above, upper shift register 104 can perform one loadfunction 114 to load eight bits in parallel at 1 GHz and perform oneshift function 116 to shift eight bits at 8 GHz all in one cycle offunction select clock CLK_(FS1). Thus, function select clocks CLK_(FS1)and CLK_(FS2) can advantageously be generated from load clock CLK_(L).For example, in the present embodiment, control circuit 202 includesfrequency divider 228, which is a divide-by-two frequency divider in thepresent example. As such, while load clock CLK_(L) has a frequency of 1GHz, function select clocks CLK_(FS1) and CLK_(FS2) have a frequency of500 MHz. Thus, during every cycle of function select clock CLK_(FS1),for example, upper shift register 104 is configured to perform one loadfunction 114 to load eight bits at 1 GHz in parallel and to perform oneshift function 116 to shift the eight bits of parallel input data P_(i1)into output register 108 at 8 GHz.

FIG. 2 shows output select clock CLK_(OS) as equal to function selectclock CLK_(FS2). As described above, output register 108 is configuredto receive shifted (serialized) data SH₁ from upper shift register 104when upper shift register 104 is performing shift function 116 and isconfigured to receive shifted (serialized) data SH₂ from lower shiftregister 106 when lower shift register 106 is performing shift function120. As such, in FIG. 1, output register 108 can be configured toalternately receive parallel input data P_(i1) from upper shift register104 as shifted (serialized) data SH₁ and parallel input data P_(i2) fromlower shift register 106 as shifted (serialized) data SH₂ withoutadditional control circuitry by utilizing one of function select clocksCLK_(FS1) and CLKF_(S2).

Thus, as described above, a serializer in accordance embodiments of thepresent invention can include a control circuit that is small and is notcomplex. For example, serializer 100 can include control circuit 202,which does not require a phase shifter to generate a signal forcontrolling selection of channels in serializer 100. Furthermore,control circuit 202 can easily be scaled when the number of channels inparallel input bus 110 is varied. For example, frequency F_(L) of loadclock CLK_(L) and frequency f_(SH) of shift clock CLK_(SH) can easily bevaried to accommodate more channels in parallel input bus 110. Also,frequency F_(L) of load clock CLK_(L) and frequency f_(SH) of shiftclock CLK_(SH) can easily be varied to accommodate for throughputrequirements of upper and lower shift registers 104 and 106 and outputregister 108. It will be appreciated that control circuit 202 is notlimited to the specific design shown.

Turning to FIG. 3, FIG. 3 shows upper and lower shift registers 304 and306 and output register 308, according to one embodiment of theinvention. Upper and lower shift registers 304 and 306 and outputregister 308 correspond respectively to upper and lower shift registers104 and 106 and output register 108 in FIG. 1. Thus, in FIG. 3, parallelinput bus 310, parallel input data P_(i1) and P_(i2), shifted(serialized) data SH₁ and SH₂, and serial output data S_(o), and serialoutput 312 correspond respectively to parallel input bus 310, parallelinput data P_(i1) and P_(i2), shifted (serialized) data SH₁ and SH₂, andserial output data S_(o) in FIG. 1. Furthermore, control circuit 202 inFIG. 2 can provide function select clocks CLK_(FS1) and CLK_(FS2,)function control clocks CLK_(FC1) and CLK_(FC2), and output select clockCLK_(OS) as shown in FIG. 3.

As shown in FIG. 3, upper shift register 304 comprises scan flip-flops330 a, 330 b, 330 c, 330 d, 330 e, 330 f, 330 g, and 330 h (alsoreferred to herein as “scan flip-flops 330”). Each of scan flip-flops330 have data inputs D_(u) and data inputs Ti_(u) and are configured tostore one bit of parallel input data P_(i1). As shown in FIG. 3, datainputs D_(u) are configured to load parallel input data P_(i1) fromparallel input bus 310. For example, data inputs D_(u) are connected toa corresponding channel of parallel input bus 310 and scan flip-flops330 can each store one bit from a corresponding data input D_(u). Alsoshown in FIG. 3, data inputs TI_(u) are configured to shift parallelinput data P_(i1) through upper shift register 304. For example, FIG. 3shows data inputs TI_(u) connected in cascade.

Similarly, lower shift register 306 comprises scan flip-flops 332 a, 332b, 332 c, 332 d, 332 e, 332 f, 332 g, and 332 h (also referred to hereinas “scan flip-flops 332”). Each of scan flip-flops 332 have data inputsD₁ and data inputs Ti₁ and are configured to store one bit of parallelinput data P_(i2). As shown in FIG. 3, data inputs D₁ are configured toload parallel input data P_(i2) from parallel input bus 310. Forexample, data inputs D₁ are connected to a corresponding channel ofparallel input bus 310 and scan flip-flops 332 can each store one bitfrom a corresponding data input D₁. Also shown in FIG. 3, data inputsTI₁ are configured shift parallel input data P_(i2) through upper shiftregister 304. For example, FIG. 3 shows data inputs TI₁ connected incascade.

FIG. 3 also shows function select clock CLK_(FS1) connected to datainput selector TE_(u) of each scan flip-flop 330. Furthermore, functioncontrol clock CLK_(FC1) is connected to a clock input of each scanflip-flop 330. Similarly, function select clock CLK_(FS2) is connectedto data input selector TE₁ of each scan flip-flop 332. Furthermore,function control clock CLK_(FC2) is connected to a clock input of eachscan flip-flop 330. Thus, control circuit 202 can manage upper and lowershift registers 304 and 306 to convert parallel input data P_(i1) andparallel input data P_(i2) into serial output data S_(o), as describedabove.

In FIG. 3, output register 308 comprises scan flip-flop 334 having datainputs D_(o) and TI_(o) and data input selector TE_(o), which can storeone bit at a time selected from shifted (serialized) data SH₁ andshifted (serialized) data SH₂. Data inputs D_(o) and TI_(o) areconnected to respective outputs of upper and lower shift registers 304and 306. Thus, data input selector TE_(o) can receive output selectclock CLK_(OS) from control circuit 202 to select between receivingparallel input data P_(i1) from upper shift register 304 as shifted(serialized) data SH₁ and parallel input data P_(i2) from lower shiftregister 306 as shifted (serialized) data SH₂. As such, output register308 is configured to alternately receive parallel input data P_(i1) fromupper shift register 304 as shifted (serialized) data SH₁ and parallelinput data P_(i2) from lower shift register 306 as shifted (serialized)data SH₂.

Thus, as described above, serializer 100 can comprise upper and lowershift registers 304 and 306 and output register 308 implemented usingall scan flip-flops. Thus, serializer 100 does not requiresynchronization or re-time flip-flops and multiplexers or othercombinational logic in the data conversion path, thereby allowing forfast conversion speed and smaller size. For example, serializer 100 canconvert parallel input data P_(i1) and P_(i2) to serial output dataS_(o) while only being limited by the performance of scan flip-flops330, 332, and 334. Furthermore, in one embodiment, scan flip-flops 330,332, and 334 comprise only 1V CMOS standard cells. Thus, serializer 100can be smaller, consume less power, and be implemented without customdesign that would increase design cost. Serializer 100 also does notrequire current mode logic (CML) for the conversion. As such, serializer100 does not require circuitry that may be required to convert from aCMOS signal to a CML signal and does not require a constant biascurrent. In one embodiment, serializer 100 has an area of 167 um² andconsumes 0.94 mW per channel. Thus, the present invention allows forserializer 100 to be single stage and scalable while being efficient inpower and area. In some embodiments, serializer 100 can operate at 8 GHzat the slow slow (SS) corner of a semiconductor wafer.

Referring to FIG. 4, FIG. 4 illustrates exemplary waveforms 400,according to one embodiment of the invention. FIG. 4 shows waveforms ofshift clock CLK_(SH), load clock CLK_(L), function select clocksCLK_(FS1) and CLK_(FS2) and function control clocks CLK_(FC1) andCLK_(FC2) corresponding respectively to shift clock CLK_(SH), load clockCLK_(L), function select clocks CLK_(FS1) and CLK_(FS2) and functioncontrol clocks CLK_(FC1) and CLK_(FC2) in FIGS. 1 through 3. Also in thepresent embodiment, output selector CLK_(OS) corresponds to outputselector CLK_(OS) in FIGS. 1 through 3 and is equal to function selectclock CLK_(FS2) in the present embodiment.

Exemplary operation of serializer 100 in FIG. 1 will be described withrespect to waveforms 400 in FIG. 4, control circuit 202 in FIG. 2, andupper and lower shift registers 304 and 306 and output register 308 inFIG. 3. In the present example, shift clock CLK_(SH) has a frequency of8 GHz and load clock CLK_(L) has a frequency of I GHz. Thus, functionselect clock CLK_(FS2) has a frequency of 500 MHz due to frequencydivider 228. Furthermore, function select clock CLK_(FS1) is the inverseof function select clock CLK_(FS2) due to inverter 226. Also, outputselector CLK_(OS) is equal to function select clock CLK_(FS2) as shownin FIG. 4.

For the purposes of the present example, each of scan flip-flops 330,332, and 334 are positive edge triggered flip-flops. However, in otherembodiments, scan flip-flops 330, 332, and 334 can be negative edgetriggered. Between times t₀ and t₁, function select clock CLK_(FS1) islow. Thus, data input selectors TE_(u) of scan flip-flops 330 areselecting for data inputs D_(u). Also between times t₀ and t₁, datainput selectors TE₁ of scan flip-flops 332 are selecting for data inputsTI₁. Thus, between times t₀ and t₁, when function control clockCLK_(FC1) transitions high, each of scan flips-flops 330 will load onebit of parallel input data P_(i1) through a corresponding data inputD_(u) in load function 114. Also between times t₀ and t₁, each timefunction control clock CLK_(FC2) transitions high, one bit of parallelinput data P_(i2) will be shifted through scan flip-flops 332. Thus,between times t₀ and t₁ scan flip-flops 332 will shift one bit perpositive edge of function control clock CLK_(FC2) thereby shiftingparallel input data P_(i2) to output register 308 in shift function 120.

Also between times t₀ and t_(1,) output selector clock CLK_(OS) is high.Thus, data input selector TE_(o) of scan flip-flop 334 is selecting fordata input TI_(o). As such, output register 308 will receive parallelinput data P_(i2) from lower shift register 306 as shifted (serialized)data SH₂. As a clock of scan flip-flop 334 is connected to shift clockCLK_(SH), between times t₀ and t₁, each time shift clock CLK_(SH)transitions high, one bit of parallel input data P_(i2) will be shiftedthrough output register 308 to serial output data S_(o). Thus, betweentimes t₀ and t₁, output register 308 will receive parallel input dataP_(i2) from lower shift register 306.

Between times t₁ and t₂, function select clock CLK_(FS2) is low. Thus,data input selectors TE₁ of scan flip-flops 332 are selecting for datainputs D₁. Also between times t₁ and t₂, data input selectors TE_(u) ofscan flip-flops 330 are selecting for data inputs TI_(u). Thus, betweentimes t₁ and t₂, when function control clock CLK_(FC2) transitions high,each of scan flips-flops 332 will load one bit of parallel input dataP_(i2) through a corresponding data input D₁ in load function 118. Alsobetween times t₁ and t₂, each time function control clock CLK_(FC1)transitions high, one bit of parallel input data P_(i1) will be shiftedthrough scan flip-flops 330. Thus, between times t₁ and t₂ scanflip-flops 330 will shift one bit per positive edge of function controlclock CLK_(FC1) thereby shifting parallel input data P_(i1) in shiftfunction 116.

Also between times t₁ and t_(2,) output selector clock CLK_(OS) is low.Thus, data input selector TE_(o) of scan flip-flop 334 is selecting fordata input D_(o). As such, output register 308 will receive parallelinput data P_(i1) from upper shift register 304. As the clock of scanflip-flop 334 is connected to shift clock CLK_(SH), between times t₁ andt₂, each time shift clock CLK_(SH) transitions high, one bit of parallelinput data P_(i1) will be shifted through output register 308 intoserial output data S_(o). Thus, between times t₁ and t₂, output register308 will receive parallel input data P_(i1) from upper shift register304. The sequence described above between times t₀ and t₂ can berepeated as desired.

Thus, as discussed above, in the embodiments of FIGS. 1 through 4, thepresent invention can provide for a serializer configured to convertparallel input data from a parallel input bus into serial output data ona serial output. As described above, in accordance with variousembodiments, the serializer can advantageously avoid various features,which would otherwise result in a slow conversion speed, high powerconsumption, and large size. Furthermore, the serializer is scalable anddoes not require substantial redesign when the number of channels of aparallel input bus is varied. Thus, the present invention can providefor a single stage and scalable serializer that is efficient in powerand area.

From the above description of the invention it is manifest that varioustechniques can be used for implementing the concepts of the presentinvention without departing from its scope. Moreover, while theinvention has been described with specific reference to certainembodiments, a person of ordinary skill in the art would appreciate thatchanges can be made in form and detail without departing from the spiritand the scope of the invention. Thus, the described embodiments are tobe considered in all respects as illustrative and not restrictive. Itshould also be understood that the invention is not limited to theparticular embodiments described herein but is capable of manyrearrangements, modifications, and substitutions without departing fromthe scope of the invention.

1. A serializer comprising: upper and lower shift registers configuredto perform a load function wherein parallel input data is loaded from aparallel input bus and a shift function wherein said parallel input datais shifted to an output register; said upper shift register configuredto perform said load function while said lower shift register performssaid shift function and said lower shift register configured to performsaid load function while said upper shift register performs said shiftfunction; an output register configured to alternately receive saidparallel input data from said upper shift register and said parallelinput data from said lower shift register.
 2. The serializer of claim 1,wherein said output register is configured to alternate betweenreceiving said parallel input data from said upper shift register andsaid parallel input data from said lower shift register.
 3. Theserializer of claim 1, wherein said upper and lower shift registers areconfigured to perform said shift function at a frequency of a shiftclock and wherein said load function of said upper and lower shiftregisters is performed at a frequency of a load clock.
 4. The serializerof claim 3, wherein said parallel input bus has a number of channels,said frequency of said load clock being equal to said frequency of saidshift clock divided by said number of channels.
 5. The serializer ofclaim 1, wherein said upper and lower shift registers are configured toperform said shift function at a frequency of a shift clock, said outputregister configured to alternately receive said parallel input data fromsaid upper shift register and said parallel input data from said lowershift register at said frequency of said shift clock.
 6. The serializerof claim 1, wherein said upper and lower shift registers are configuredto perform said load function and said shift function by selectivelyreceiving a load clock and a shift clock.
 7. The serializer of claim 1,wherein said upper and lower shift registers are configured to selectbetween performing said load function and said shift function responsiveto a respective function select clock.
 8. The serializer of claim 7,wherein said respective function select clock of said upper shiftregister is an in inverse of said respective function select clock ofsaid lower shift register.
 9. The serializer of claim 7, wherein saidoutput register is configured to select between receiving said parallelinput data from said upper shift register and said parallel input datafrom said lower shift register responsive to one of said respectivefunction select clocks.
 10. The serializer of claim 1, wherein saidupper and lower shift register comprise scan flip-flops, said scanflip-flops having first and second data inputs, said first data inputsare configured to load said parallel input data from said parallel inputbus and said second data inputs are connected in cascade.
 11. Theserializer of claim 1, wherein said output register is a flip-flop. 12.The serializer of claim 1, wherein said output register is a scanflip-flop having a first data input configured to receive said parallelinput data from said upper shift register and a second data inputconfigured to receive said parallel input data from said lower shiftregister.
 13. A serializer comprising: an upper shift registercomprising scan flip-flops, each of said scan flip-flops having firstand second data inputs, said first data inputs configured to load firstparallel input data from a parallel input bus and said second datainputs configured to shift said first parallel input data through saidupper shift register; a lower shift register comprising scan flip-flops,each of said scan flip-flops having first and second data inputs, saidfirst data inputs configured to load second parallel input data fromsaid parallel input bus and said second data inputs configured to shiftsaid second parallel input data through said lower shift register; anoutput register configured to alternately receive said first parallelinput data from said upper shift register and said second parallel inputdata from said lower shift register.
 14. The serializer of claim 13,wherein said output register is configured to alternate betweenreceiving said first parallel input data and said second parallel inputdata.
 15. The serializer of claim 13, wherein said second data inputs ofsaid upper shift register are connected in cascade and said second datainputs of said lower shift register are connected in cascade.
 16. Theserializer of claim 13, wherein said output register is a flip-flop. 17.The serializer of claim 13, wherein said output register is a scanflip-flop having a first data input configured to receive said firstparallel input data from said upper shift register and a second datainput configured to receive said second parallel input data from saidlower shift register.
 18. The serializer of claim 13, wherein said uppershift register is configured to select between loading said firstparallel input data from said parallel input bus and shifting said firstparallel input data responsive to a function select clock.
 19. Theserializer of claim 13, wherein said upper shift register is configuredto shift said first parallel input data at a frequency of a shift clock,said output register configured to alternately receive said firstparallel input data from said upper shift register and said parallelinput data from said lower shift register at said frequency of saidshift clock.
 20. The serializer of claim 13, wherein said scanflip-flops comprise complementary metal oxide semiconductor (CMOS)standard cells.